Ankylosaurus magniventrishas always been my favorite. Huge (33' and around 8 tons) and one of the spikiest of all ankylosaurs. My favorite dinosaur of all time.
I prefer the theory that it's both false head and weapon. Dinosaurs often didn't have great vision so a false head doesn't have to be very convincing. Getting a predator to attack the club would put it in perfect striking range, the tails weren't as flexible as crocodiles or iguanas, and 8 ton giant bearded dragons don't spin on a dime.
*promote a *quality beast.